Apple stock (AAPL) was trading at $475.77 mid-afternoon after 2 pm. At 2:12 pm, Carl Icahn’s twitter account announced, “We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come.”

He then followed up at 2:25 pm that he “Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done now. We plan to speak again shortly.”

The stock then rose to a high of $494.66 before settling at the close at $489.57. This is an increase of $18.63, or about 4% higher from its opening price of $470.94. As Philip Elmer-DeWitt of CNNMoney’s blog Apple 2.0 reports, that’s roughly a $17 billion gain in market value in about an hour. Apple also verified that Carl Icahn had been in touch with them, saying “We appreciate the interest and investment of all our shareholders. Tim had a very positive conversation with Mr. Icahn today.”

The fact that this man’s tweets can move the entire stock market through Apple is incredible. The NASDAQ was up 14.49 points, or 0.39% for the day.